docs: def do_proxy_failover(self, proxy_url, for_url):
          """
          :param str proxy_url: Proxy to ban.
          :param str for_url: The URL being requested.
          :returns: The next proxy config to try, or 'DIRECT'.
          :raises ProxyConfigExhaustedError: If the PAC file provided no usable proxy configuration.
          """
          self._proxy_resolver.ban_proxy(proxy_url)
          return self._proxy_resolver.get_proxy_for_requests(for_url)